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People who start smoking marijuana before they turn 16 may damage their brains more than people who start later, according to a small study from McLean Hospital released yesterday. Early-onset users also smoke more marijuana and more often over the course of a week than later-onset users, the researchers found.

Neuroscientists had previously shown that people who regularly smoke large amounts of marijuana do not do well on tests of memory and other mental abilities.

Staci Gruber, director of the Cognitive and Clinical Neuroimaging Core at McLean in Belmont and assistant professor of psychiatry at Harvard Medical School, led a team that tested mental focus and flexibility among 33 young marijuana users and 26 non-users recruited from Greater Boston.

“Age really does matter,’’ she said. “Early onset is related to more frequent and higher magnitude of use and significantly greater impairment on these tasks, which ultimately could result in greater difficulty with everyday life activities and decision-making.’’

One test asked subjects to sort a deck of cards following a set of rules that changed without warning. Early-onset marijuana smokers did significantly worse than both smokers who began their regular marijuana use after age 16 and nonsmokers. When mistakes were pointed out to all the subjects as they went along, early users continued to make the same errors.

The other test assessed how well the subjects dealt with distraction during a number recognition task. Shown a series of three numbers, they were asked to identify the number that was never the same as its position in the set of three numbers. For example, the number 2 was never the second in a string of three numbers. Early-onset smokers made nearly twice as many errors as later-onset smokers. The more grams of marijuana they smoked per week, the worse they did.

Taken together, both tests revealed that the early-onset smokers were not able to adjust their thinking on the fly or to focus their attention on a task, Gruber said in an interview before presenting the results to the Neuroscience 2010 conference in San Diego. The study, which will be submitted to a scientific journal in the next few weeks, has not been peer-reviewed, a process designed to ensure published research meets scientific standards.

Early-onset users smoked three times as much marijuana per week and twice as often, compared with later-onset users. Younger brains are more vulnerable to drugs and alcohol, Gruber said, making proposals for legalizing marijuana troubling if safeguards are not put in place for young people, whose brain development continues into their 20s.

“If we know the developing brain is sensitive to drugs and we see a direct relationship between early exposure to marijuana and [cognitive] performance, we have to let people know we need to perhaps impose guidelines, like you can’t do this until you’re 21, [as with] cigarettes and drinking,’’ she said.